This invention disclosure relates to the processing of marine seismic data, and in particular, to wave field decomposition in marine seismic data.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. In a typical marine seismic survey, one or more marine seismic streamers are towed behind a survey vessel. The seismic streamers may be several thousand meters long and contain a large number of sensors, which are distributed along the length of the each seismic streamer cable. The survey vessel also includes one or more seismic sources, such as airguns and the like.
As the streamers are towed behind the survey vessel, acoustic signals, commonly referred to as “shots,” produced by the seismic source are directed down through the water column into strata, beneath a water bottom surface, where they are reflected from the various subterranean geological formations travel back to sea surface (up-going wave). One well known problem in marine seismic is that, up-going waves are reflected once more at the sea surface because of the air-water interface. Hence the sensors in the seismic streamer cable record not only the desired wave field (up-going wave, i.e., reflected signal from various subterranean geological formations) but also their reflections from the sea surface (down-going wave) because of the air-water interface. This undesired term is known as “ghost” in the art. Depending on the incidence angle of the up-going wave field and depth of the streamer cable, the interference between the up-going and down-going wave fields create nulls or notches in the recorded spectrum. These notches reduce the useful bandwidth of the spectrum and limits the possibility of towing the streamer in deep water (e.g., at 20 m).
The process of decomposing the recorded wave field into up-and down-going components is known as wave field separation or deghosting in the literature. It is known that, to this purpose, particle velocity sensors can be incorporated into the streamer in addition to the pressure sensors. Then by combining the pressure and particle velocity measurements, the “ghost” free data, known as up-going wave field can be calculated. In the case of vertical incidence, the standard method to do the wave field separation is to add and subtract a scaled version of the vertical component of the particle velocity measurement to and from pressure measurement. This standard technique is known as PZ-summation in the literature.
There exist several methods which attempt to solve the wave field separation problem. The major problem with existing techniques of combining pressure and particle velocity measurements is that they do not exploit the statistics of the measurement noises to achieve optimal wave field separation. For instance some methods ignore the data from particle motion sensors at low frequencies (e.g., frequencies below 20 Hz) arguing that they are too noisy. Then they do single streamer deghosting using only pressure measurements at these frequencies. At higher frequencies, they do the standard PZ-summation.
In this invention, we solve the optimal wave field separation problem when a vector of measurements is obtained by using a towed streamer. Depending on the set of available data the method solves both 1-D, 2-D and 3-D wave field separation problem.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.